Bridge rectifier operation and power factor correction circuit

ABSTRACT

A power factor correction “PFC” circuit for use in AC-DC conversion comprises first and second pairs of switches connected across input and output rails in a full bridge configuration, and a controller configured to control the operation of the switches such that one pair of switches is switched on and off at an AC input frequency and the other pair of switches is switched on and off at high frequency; and the switching frequency is alternated between the pairs of switches every n cycles of the AC input/output frequency. The circuit may include inductors on both input rails and may be used in a bidirectional power supply.

CROSS REFERENCE TO RELATED APPLICATIONS

A claim for priority under 35 U.S.C. § 119 is made to United KingdomPatent Application No. 2010275.2, filed on Jul. 3, 2020, the entirecontents of which are hereby incorporated by reference.

The disclosure relates to the field of power convertor circuits such asAC/DC.

BACKGROUND

In power supply systems, such as AC/DC conversion systems, it is knownto include power-factor corrector “PFC” circuitry to improve theefficiency of converting AC to DC. Such circuitry may comprise a dioderectifier bridge.

A particular example of PFC circuitry is the so-called BridgelessTotem-Pole PFC, or BTP-PFC. In the BTP-PFC topology, one half of a dioderectifier bridge is replaced by active switches in a half bridgearrangement. A pair of diodes and the pair of active switches then forma full bridge. The diodes operate at the AC line frequency and theactive switches are switched at a higher frequency, usually at least tentimes the AC line frequency, commonly referred to as high frequencyswitching. The diodes may also be replaced by switches in order toimprove efficiency, such as MOSFETs, but due to the differentfrequencies it is usual for the diodes and the active switches to beformed from different components which can then be optimised for thefrequency or any other differences in operating conditions between thediodes and the switches.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the detaileddescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to determine the scope of the claimed subject matter.

There is provided in the following a method of operating a full bridgerectifier, comprising first and second pairs of switches, for AC to DCconversion. In the method, one pair of switches is switched at the ACline frequency, the other pair of switches is switched at highfrequency, and the switching frequency is alternated between the pairsof switches every n cycles of the AC frequency. In other words, theswitching frequency of each switch is changed from high frequency to theAC input frequency or vice versa after every n cycles. Thus, the rolesof the switches may be reversed between AC cycles.

As a result of the alternation of the switching frequency between onewhole cycle and the next (or between n whole cycles and the next n wholecycles) it is possible, but not essential, for the same components to beused at all four switch locations of the rectifier topology since anythermal or other energy losses in the switches are equalised over 2ncycles. For example, high quality MOSFET switches that are capable offast switching speeds may be used in all four switch locations of thefull bridge. The alternation of the switching frequency helps to avoidsome components aging faster than others. Without this alternation, iffor example high quality MOSFETs were used in all 4 locations (both“legs” of the bridge), there would be unequal thermal loss, and giventhe same thermal interface, one pair of switches (one bridge leg) wouldrun hotter than the other. The alternation of the roles of the switchesallows equal distribution of the thermal loss between all four switchingdevices yielding improved performance, increased reliability andsimplified thermal design.

Embodiments of the invention may comprise manipulation of switchingpatterns, for example by a controller, e.g. digital controller, tochange which pair of switches sees the lower loss that is experienceddue to AC line frequency switching. By doing so, the loss in either pairof switches can be controlled, and the temperature of any one pairminimised. As noted above this can (but not necessarily) then allow theuse of the same switching devices in all four locations, offeringgreater reliability because they are cooler and potential cost savings.

Here “n” may be any positive integer, usually but not necessarily 1.

This method of rectifier operation may be implemented in a PFC circuitand embodiments of the invention therefore provide a PFC circuit. In thePFC circuit an inductor is usually provided on at least one or both ofthe input rails which may be configured to operate in conjunction withthe switches as a boost convertor.

The PFC may also be used for DC-AC conversion and thus some embodimentsof the invention provide a bidirectional power supply. Here the presenceof an inductor on both rails is particularly useful in reducingelectromagnetic interference. This is a problem with the use of theknown BTP-PFC topology in a bidirectional power supply.

Features of different aspects and embodiments of the invention may becombined as appropriate, as would be apparent to a skilled person, andmay be combined with any of the aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example onlyand with reference to the following drawings, in which:

FIG. 1 is a diagram of a full bridge rectifier suitable for use in someembodiments of the invention;

FIGS. 2A, 2B, 2C, and 2D are circuit diagrams showing a practicalimplementation of the circuit of FIG. 1 according to some embodiments ofthe invention;

FIG. 3 is a graph showing four half cycles of an AC input voltage toillustrate the operation of the circuit of FIGS. 2A-2D;

FIG. 4 is a circuit diagram of a power factor correction circuitaccording to some embodiments of the invention.

Common reference numerals are used throughout the figures to indicatesimilar features.

DETAILED DESCRIPTION

Embodiments of the present invention are described below by way ofexample only. These examples represent the best ways of putting theinvention into practice that are currently known to the applicantalthough they are not the only ways in which this could be achieved.

FIG. 1 is a diagram showing a basic rectifier circuit for the purpose ofexplaining embodiments of the invention. The circuit may be used in bothrectification (AC-DC) and inversion (DC-AC) circuits. In the following,AC-DC is described. It will be understood that for DC-AC the inputterminals become output terminals and vice versa.

The circuit of FIG. 1 comprises a pair of DC output rails R1, R2 withoutput terminals T1, T2 and a pair of AC input rails R3, R4 with inputterminals T3, T4. First and second pairs of unidirectional switches,e.g. diodes, S1, S2 and S3, S4 are connected between the terminalsacross the rails in a full bridge configuration. Thus current can flowin one direction from T3 to R2 via S4 and from R1 to T4 via S1, or inthe other direction from T4 to R2 via S2 and from R1 to T3 via S3. Anenergy storage capacitor C1 is connected between the DC rails. Eachswitch pair, S1,S2 or S3,S4, forms a half bridge, also referred to inthe art as a bridge “leg” between a pair of terminals, in this case theDC terminals T1, T2.

In known AC-DC conversion circuits, one pair of switches is turned offand on at high frequency and an energy storage device such as capacitorC1 may supply power during the non-conduction state of the switches.Thus for example switches S1 and S2 would be switched at high frequency,and switches S3 and S4 would be switched at the desired AC outputfrequency. Therefore it would be typical for switches S1 and S2 to bedifferent kinds of switches from S3 and S4, as in the example of theBTP-PFC discussed above. As noted above, the high frequency is at leastten times or higher (orders of magnitude higher) than the desired outputAC frequency, e.g. at least 500 Hz for 50 Hz AC output. By switching athigher and higher frequencies in a PFC, or other AC-DC conversioncircuit, the size of the parts can be reduced so that the volumetricefficiency is improved. Therefore, more commonly, the high frequency isat least 10 kHz and may be as high as 10 MHz.

According to some embodiments of the present invention, the switchingfrequency is alternated between the pairs of switches every n cycles ofthe AC frequency. Thus, the roles of the switches S1-S4 may bedynamically allocated. It is well known that a switching operationinherently incurs energy loss, especially thermal energy, in particularso called “edge loss” at the edge of a switch pulse as the state of theswitch is changed. One effect of the alternation, or dynamic allocationof switching frequency, is to even out the energy loss across all fourswitches. If the switching frequencies are changed from high frequencyto the AC input frequency or vice versa for each switch after every ncycles, where n is an integer, then after 2×n cycles of the desiredoutput frequency each of the switches will dissipate the same amount ofpower or thermal energy

It will be appreciated that a PFC circuit according to some embodimentsof the invention may additionally comprise a controller, for example adigital controller, not shown in FIG. 1 , to control the operation ofthe switches in order to achieve the alternation of switching frequency.

A suitable value for n is 1, and this will be used in the followingdescription. When n=1, the switching frequency of each switch willchange with alternate cycles which are therefore referred to as “odd”and “even” cycles for convenience, the designations “odd” and “even”being assigned arbitrarily. However it should be borne in mind that nmay be any integer, in which case the odd and even cycles will becomealternate occurrences of n consecutive cycles.

Each switch shown in FIG. 1 may be formed from more than one component,for example multiple switching devices in parallel used to accommodatehigher current flows. Therefore references to “switch” as used herein,unless otherwise stated, include multiple devices operating together,for example under the same control signal or switching pattern.

A practical implementation of the circuit of FIG. 1 will now bedescribed with reference to FIGS. 2A-2D and FIG. 3 . These figures showpart of a PFC circuit according to some embodiments of the invention indifferent states over four half cycles of the AC input voltage, shown inFIG. 3 . As noted above, the switching frequency is alternated betweenthe pairs of switches every n cycles of the AC frequency. FIGS. 2A-2Dand FIG. 3 show an example where n=1.

In the circuits shown in FIGS. 2A-2D, transistors TR11, TR12, TR13 andTR14 correspond respectively to switches S1-S4 of FIG. 1 and capacitorC17 corresponds to capacitor C1 of FIG. 1 . Transistors that are not inan ON state for any half cycle are not shown for the sake of clarity. InFIG. 3 , consecutive cycles are denoted odd and even. FIG. 2A shows thestate of transistors TR11-TR14 in an odd positive half cycle, FIG. 2Bthe following odd negative half cycle, FIG. 2C the following evenpositive half cycle and FIG. 2D the following even negative half cycle.

In the odd positive half cycle as shown in FIG. 2A, transistor TR12 isON for the whole of the half cycle and transistors TR11 and TR13 areswitched at high frequency. In the odd negative half cycle as shown inFIG. 2B transistor TR14 is ON for the whole of the half cycle andtransistors TR11 and TR13 continue to be switched at high frequency. Inthe even cycle the roles of the transistors are reversed as shown inFIGS. 2C and 2D. TR14 and TR12 are switched at high frequency and TR11and TR12 are ON for full positive and negative half cycles respectively.

The controlling signals for the transistors TR11 to TR14 are applied tothe G terminals with respect to the S terminals, indicated as gate andsource blocks GA, SA, for TR11; GB, SB, for TR13; GC, SC for TR12 andGD, SD for TR14. The switching frequencies, or the roles of therespective switches, in this example transistors TR11-TR14, can beachieved through the drive pattern to the respective switches, i.e. thegate drive pattern in the case of FETs. The switching frequencies of therespective switches may be controlled by a digital controller, not shownin FIGS. 2A-2D, which may also control the duty cycles of the respectiveswitches.

In all of the embodiments described here, the four switches comprisingthe two pairs of switches forming the bridge legs, e.g. transistors, maybe formed from identical or similar components. For example they may allbe rated for the high switching frequency. Any suitable switchingcomponents may be used, for example but not limited to MOSFETs,insulated-gate bipolar transistors “IGBTs”, and silicon carbideswitches. GaN devices are examples of MOSFETs having high switchingspeed capability.

According to some embodiments of the invention an inductor may beprovided on one or both of the input rails. The inductor or inductorstypically operate, in conjunction with the high frequency switched pair,as a boost convertor. In the illustrated embodiments the inductors havethe same inductance and may comprise two separate identical componentswhich may share a common magnetic core.

The partial circuits shown in FIGS. 2A-2D also include inductors L2 andL4 on the respective AC input rails between input terminals AC_L1, AC_L2and the respective bridge legs, or the junctions between the respectiveswitch pairs. The two inductors, in conjunction with the high frequencyswitch pair, operate as a boost convertor, as is known in the art, toachieve a higher DC output voltage than the peak AC input voltage. In aconventional boost convertor, a diode or other one way device connectsthe inductor to the output and a switch connects the inductor to theopposite input terminal. In the circuit of FIGS. 2A-2D, two lineinductors are not essential for the circuit to operate as a boostconvertor, and either of the inductors could be omitted.

As shown in FIGS. 2A-2D, the roles of the respective switches alternatebetween boost switch and boost diode between successive half cycles andtherefore, as is known in the operation of boost convertors, the ON/OFFduty cycle of the switch pair being switched at high frequency willalternate from one half cycle to the next, i.e. from the positive halfcycle to the negative and vice versa. Therefore, to take TR11 as anexample, the role of this switch changes from boost switch, to boostdiode, to OFF and then to ON over four half cycles of the input ACvoltage frequency.

More generally, in the circuit shown in FIGS. 2A-2D the switch, e.g.transistor, function is allocated dynamically according to either odd oreven input, e.g. mains, cycle (determined arbitrarily) and to positiveor negative voltage (determined already by circuit function). The rolesof the switches alternate between boost diode and boost switch from thepositive AC half cycle to the negative AC half cycle or from thenegative AC half cycle to the positive half cycle, or both if n isgreater than 1.

In an alternative embodiment where n is greater than 1, the role of oneswitch would change from boost switch to boost diode after each halfcycle, e.g. each transition from positive to negative or vice versa,i.e. change in polarity, for n half cycles and then from ON to OFF aftereach half cycle for the next n half cycles.

The circuits described herein may also be used in DC-AC conversion, i.e.in inverters, as will be described further below. Thus they may formpart of a bidirectional power supply. The inclusion of an inductor oneach of the input rails, rather than on only one rail, aids theelectromagnetic interference (EMI) performance of the circuit in bothAC-DC mode and in DC-AC mode. Neither of inductors L2 and L4 is requiredunless boost or buck (see below) conversion is to be achieved, and forboost conversion only one may be provided. For either boost or buckfunction a minimum of one inductor L2 or L4 is required. However, toaid, i.e. reduce, EMI two inductors L2 and L4 may be used.

As noted elsewhere here, the circuits shown in FIGS. 1 and 2 may beimplemented in a power factor correction circuit, for example as part ofan AC-DC convertor.

FIG. 4 shows a power factor correction circuit in further detail,according to some embodiments of the invention. The circuit of FIG. 4may be operated as part of a bidirectional power supply and thereforemay operate as a rectifier or as an inverter.

Some general points in connection with the circuit of FIG. 4 should benoted. Sampling functions may be provided, as known in the art, tofacilitate full closed loop control of output voltages. These are notshown in FIG. 4 . R44 has been added to provide a reference point, wherethe input and output voltages may be measured (for the purposes ofclosed loop control) with respect to.

As noted above, the switching frequency is typically at least ten timeslarger than the AC input frequency. The same applies when the circuit isoperated as an inverter—the switching frequency is very much higher thanthe AC output frequency. The filter cut-off frequency of the two stagefilter formed by L1, L3 & C49 (1st stage) and L2, L4 & C50 (2nd stage)is higher than the line in/out frequency but lower than the switchingfrequency. A two stage filter is not essential and the same can beachieved with one filter stage. Therefore L2, L4 and C50 may be omitted.

To handle high power loads it may be necessary to use multiple switchingdevices in parallel to permit them to current share. The circuit of FIG.4 shows two GaN switching devices in parallel at each switch location ofthe bridge rectifier, namely Q5/Q7, Q9/Q11 forming a pair correspondingto the pair S1, S2 of FIG. 1 or pair TR11, TR13 of FIGS. 2A-2D, andQ6/Q8, Q10/Q12 corresponding to the pair S3,S4 of FIG. 1 or TR12, TR14of FIGS. 2A-2D. When reference is made in the description of FIG. 4 to aspecific transistor the lower numbered one of a paralleled pair will bereferenced. It can be assumed that the other parallel device will ALWAYSbe in the same state (ON or OFF) as the referenced device.

DC to AC Conversion:

The circuit of FIG. 4 operates to convert DC to AC in generally the samemanner as a synchronous buck convertor, as known to those skilled in theart, to step down the input DC voltage. However embodiments of theinvention are not limited to step down DC to AC conversion.

The circuit of FIG. 4 when operated for DC to AC conversion differs froma known synchronous buck convertor in that the output filter comprisestwo inductors L1 and L3, and additional switches, transistor switches asshown in FIG. 4 , to enable the voltage to be driven both positive andnegative in order to generate an AC voltage with respect to a DC supplymidpoint.

The resultant AC output voltage zero crossing value will be positionedat exactly half-way up the DC supply voltage. For this reason the DCrails on the left in FIG. 4 are labelled as P130V and N130V so that theDC midpoint (essentially 0 V DC) is the same as the AC zero crossingpoint (0 V AC is achieved when the duty ratio of the transistors isexactly 50%).

The DC power source (whose DC value is higher than the peak of the ACsinusoid to be generated) is applied to the P130V and N130V ports.Capacitor C48 provides local energy storage such that the DC supplycurrent is smoothed out somewhat as the AC current rises and falls inconcert with the AC output voltage. Capacitors C40-43 together withresistor R40 and C54-57 together with resistor R43 provide localdecoupling for the switching transistors Q5-Q12.

Inductors L1, L3 and capacitor C49 form a low pass filter. Capacitor C46and resistor R41 form a damping network to prevent inductors L1, L3 andcapacitor C49 ringing. Inductors L2, L4 and capacitor C898 form a secondlow pass filter to further attenuate switching frequency ripple.Capacitor C47 and resistor R42 form a second damping network. RV1 andRV2 are varistors which act to clamp high voltages generated by thefilter inductors L1, L3 in the event that the load is disconnectedsuddenly. Transistor T1 and associated capacitors C51, C44, C45, C52,C53 form a common-mode EMI filter (whose operation is not relevant tothe application here).

A controller as known in the art provides source and gate drive signalsto source drive blocks SA, SB, SC, SD and gate drive blocks GA, GB, GC,GD. The controller could be any suitable controller including but notlimited to a microprocessor, digital signal processor “DSP”, applicationspecific integrated circuit “ASIC”, dedicated control integrated circuitor as shown in FIG. 4 a field-programmable gate array “FGPA”.

In the case of an FGPA controller and the transistors being GaN devices,the gate drive blocks GA, GB, GC, GD serve to boost the logic levelsignals generated by the FPGA controller to drive levels adequate forthe GaN devices whilst also providing galvanic isolation. Their primarycharacteristics are:

-   -   High current gain    -   Low propagation delay    -   Fast rise and fall times    -   Galvanic isolation    -   Controlled turn-on and off voltages.

The digital controller creates Pulse Width Modulated signals to drivethe transistors ON and OFF at a predetermined switching frequency andwith a duty ratio governed for example by a closed loop controller.Opposing switches of each pair are switched with a first duty cycle andthe other two switches are switched with a second duty cycle, and thefirst and second duty cycles of the switches are alternated for eachchange in polarity of the AC voltage. An example of the switching is asfollows: For the positive half cycles (when AC_L1 is positive withrespect to AC_L2) then Q6 and Q9 switched on and off at high frequencyso as to be driven ON for >50% of the switching period and therefore Q5and Q10 are switched on and off at high frequency to be driven ON for<50% of the switching period. Similarly for the negative half cycles(when AC_L2 is positive with respect to AC_L1) then Q5 and Q10 aredriven ON for >50% of the switching period and therefore Q6 and Q9 aredriven ON for <50% of the switching period.

The LC filters (L1, L3, C48, C49 described above) average out theapplied rectangular waveform to produce a smoothed voltage. A digitalsinewave reference is created and a sample of the filtered outputvoltage is fed back to a proportional-integral “PI” orproportional-integral-differential “PID” controller block to providefull closed-loop control of the output voltage. This is well known inthe art and the control block is not included in the figures. Because ofthe closed loop the duty ratio of the transistors ON time is controlledwhich creates a sinusoidal output voltage.

In this mode all transistors operate with the same average power (whenaveraged over a full output cycle).

AC to DC Conversion:

The circuit of FIG. 4 may be used for AC-DC conversion. In this powerconversion mode the circuit operates with the same essentialcharacteristics as a boost converter, as noted in connection with FIGS.2A-2D. Again it should be noted here that embodiments of the inventionare not limited to boost or step-up conversion.

There are some notable differences between the circuit of FIG. 4operating as a convertor and a known boost convertor: the boost inductorcomprises inductors L1, L3 on the output rails and there are additionaltransistor switches to create the full-bridge topology, for examplesimilar to the Bridgeless Totem Pole Power Factor Corrector.

The same digital controller as for DC to AC conversion, the FPGA in FIG.4 , may also create Pulse Width Modulated signals to drive thetransistors. However, for this topology there are two control loops asis known in AC-DC conversion, and specifically PFC: an inner currentloop (bandwidth approximately 1/10th the switching frequency) whichforces the line current to follow the same shape as the line voltage(the line voltage is used as a reference) and an outer voltage loop(very bandwidth limited) which forces the output voltage to be correctwhen averaged over several tens of line frequency cycles (this actionallows the converter to account for changes in output loading). An ACvoltage is applied at AC_L1 and AC_L2 and the resultant DC outputvoltage appears at P130V and N130V. In the case of a boost convertor,that the DC output voltage will always be higher than the peak of theapplied AC due to the boost function in the PFC.

In a conventional AC-DC convertor, one of the half-bridges (e.g. formedfrom Q5/Q9) would be formed using GaN devices or other devices havingexcellent high speed properties, and the other half-bridge (Q6/Q10)would be formed from conventional silicon devices where speed is notimportant. In a typical operating mode, the low speed silicon deviceswould be commutated once every 10 ms (50 Hz) or 8.3333 ms (60 Hz). TheGaN devices would be switched at high speed (100 kHz or more) and theinner current loop would force the line current to be the same shape asthe line voltage. Inductors L1 and L3 form the inductive element of aboost converter to step the output voltage up to a level higher than thepeak of the applied line voltage. L2 and L4 form an EMI/ripple filter.

According to some embodiments of this invention, all fours switchingpositions of the bridge rectifier may be occupied by the same kinds ofswitching device, for example identical devices, such as GaN devices. Aswitching device (one of the four depending upon the instantaneouspolarity of the applied AC waveform and whether the cycle is(arbitrarily) an even or odd numbered cycle) is still turned ON for halfthe mains cycle (low frequency operation). Its opposite counterpart isalways OFF for the entire half cycle. The other two switching devicesmay operate at high speed to create the sinusoidal current waveform.Their roles may also change from boost switch to boost diode or viceversa at the zero-crossing. Then for the next mains cycle the high speedand low speed device roles are swapped. Thus over two mains cycles thethermal dissipation in the four devices is essentially the same(assuming no load or line changes across the two cycles). As notedabove, the role change may occur every n cycles and need not changeafter every whole cycle.

Some embodiments of the invention may be implemented in an existingrectifier circuit by a suitable method of operation, for exampleapplication of suitable digital control signals. Thus the invention insome aspects may be regarded as a digital control method or technique.The method may be summarised as:

A method of operating a rectifier comprising first and second pairs ofswitches in a bidirectional power supply comprising:

-   -   for AC-DC conversion:    -   switching on and off both switches of the first pair of switches        at the AC line frequency,    -   switching on and off both switches of the second pair of        switches at high frequency, and    -   alternating the switching frequency between the pairs of        switches every n cycles of the AC frequency.    -   The same circuit may be used for DC-AC conversion as is known in        the art:    -   switching opposing switches in each pair with a first duty cycle        and the other switches in each pair with a second duty cycle,        and    -   alternating the first and second duty cycles of the switches for        each change in polarity of the AC output voltage.

It will be understood that the benefits and advantages described abovemay relate to one embodiment or may relate to several embodiments. Theembodiments are not limited to those that solve any or all of the statedproblems or those that have any or all of the stated benefits andadvantages.

Any reference to “an” item or “piece” refers to one or more of thoseitems unless otherwise stated. The term “comprising” is used herein tomean including the method steps or elements identified, but that suchsteps or elements do not comprise an exclusive list and a method orapparatus may contain additional steps or elements.

Further, to the extent that the term “includes” is used in either thedetailed description or the claims, such term is intended to beinclusive in a manner similar to the term “comprising” as “comprising”is interpreted when employed as a transitional word in a claim.

FIGS. 2A-2D and 4 include component values for the purpose ofillustration of embodiments of the invention and it will be appreciatedthat different component values may be used in other embodiments.

The figures illustrate exemplary methods. While the methods are shownand described as being a series of acts that are performed in aparticular sequence, it is to be understood and appreciated that themethods are not limited by the order of the sequence. For example, someacts can occur in a different order than what is described herein. Inaddition, an act can occur concurrently with another act. Further, insome instances, not all acts may be required to implement a methoddescribed herein.

It will be understood that the above description of a preferredembodiment is given by way of example only and that variousmodifications may be made by those skilled in the art. What has beendescribed above includes examples of one or more embodiments. It is, ofcourse, not possible to describe every conceivable modification andalteration of the above devices or methods for purposes of describingthe aforementioned aspects, but one of ordinary skill in the art canrecognize that many further modifications and permutations of variousaspects are possible. Accordingly, the described aspects are intended toembrace all such alterations, modifications, and variations that fallwithin the scope of the appended claims.

What is claimed is:
 1. A power factor correction “PFC” circuit for usein AC-DC conversion comprising first and second pairs of switchesconnected across input and output rails in a full bridge configuration,and a controller configured to control an operation of the switches suchthat: one pair of the first and second pairs of switches is switched onand off at an AC input frequency of an AC input signal and the otherpair of the first and second pairs of switches is switched on and off athigh frequency; and a switching frequency is alternated between thefirst and second pairs of switches every n cycles of the AC inputfrequency where n is an integer; wherein an ON/OFF duty cycle of thepair being switched at high frequency of the first and second pairs ofswitches is alternated between one half cycle of the AC input signal anda next half cycle.
 2. The PFC of claim 1 wherein four switches formingthe first and second pairs of switches are formed from components ratedfor switching at the high frequency.
 3. The PFC of claim 2 wherein thefour switches are formed of identical components.
 4. The PFC circuit ofclaim 1 comprising an inductor on one or both of AC input rails.
 5. ThePFC of claim 4 wherein the inductor is configured to operate, inconjunction with the pair being switched at high frequency, as a boostconvertor.
 6. The PFC of claim 5 wherein the controller is configured tocontrol the operation of the pair being switched at high frequency sothat a first switch of the pair operates as a boost diode and a secondswitch of the pair operates as a boost switch, with roles of the firstswitch and the second switch being reversed with a change in polarity ofthe AC input signal.
 7. A bidirectional power supply comprising a PFC asclaimed in claim
 1. 8. The bidirectional power supply of claim 7comprising an inductor on each input rail of AC input rails.
 9. Thebidirectional power supply of claim 8 wherein an inductance of theinductors on each input rail is the same.
 10. The bidirectional powersupply circuit as claimed in claim 7 wherein the controller isconfigured to operate four switches comprising a first pair of switchesincluding a first switch and a second switch and a second pair ofswitches including a third switch and a fourth switch for DC-ACconversion by switching opposing switches in each pair including thesecond switch and the third switch with a first duty cycle and the otherswitches in each pair including the first switch and the fourth switchwith a second duty cycle, and alternating the first and second dutycycles of the opposing switches and the other switches for each changein polarity of an AC output voltage.
 11. A method of operating a bridgerectifier comprising first and second pairs of switches, for AC to DCconversion, wherein one pair of the first and second pairs of switchesis switched at an AC line frequency, the other pair of the first andsecond pairs of switches is operated at high frequency, and a switchingfrequency is alternated between the first and second pairs of switchesevery n cycles of the AC line frequency where n is an integer; whereinan ON/OFF duty cycle of the pair being switched at high frequency of thefirst and second pairs of switches is alternated between one half cycleof the AC input and a next half cycle.
 12. The method of claim 11wherein an inductor is provided on one or both AC input lines to abridge rectifier and the pair of switches being switched at the highfrequency is operated with the inductor as a boost convertor.
 13. Themethod of claim 12 wherein for the pair of switches being switched atthe high frequency, the roles of the switches alternate between a boostdiode and a boost switch from one of a positive AC half cycle to anegative AC half cycle and the negative AC half cycle to the positive AChalf cycle.
 14. A method of operating a rectifier comprising first andsecond pairs of switches in a bidirectional power supply comprising: forAC-DC conversion: switching one pair of the first and second pairs ofswitches at an AC input frequency, operating the other pair of the firstand second pairs of switches at a high frequency, and alternating the ACinput frequency and the high frequency between the first and secondpairs of switches every n cycles of the AC input frequency where n is aninteger; for DC-AC conversion: the first pair of switches including afirst switch and a second switch and the second pair of switchesincluding a third switch and a fourth switch, switching opposingswitches in each pair including the second switch and the third switchwith a first duty cycle and the other switches in each pair includingthe first switch and the fourth switch with a second duty cycle, andalternating the first and second duty cycles of the opposing switchesand the other switches for each change in polarity of an AC outputvoltage.